Magnetic firing circuit for thyristors

ABSTRACT

A novel control circuit for constant current applications. The circuit comprises a thyristor bridge which is fired by a magnetic amplifier. The DC output current of the thyristors flows through the feedback winding of the magnetic amplifier to cancel the effect of the input ampere-turns, and to keep the output current constant through the equal ampere-turns characteristic of the conventional saturable reactor. Because thyristors are used in the output stage, the new circuit can control a large amount of output power with a device of light weight and small size. The principle of the new circuit is also effectively applied to parallel operation of multithyristors, in which individual currents can be adjusted to various desired values.

United States Patent [72] Inventor Kosuke l-larada Fukuoka Prefecture, Japan [21] Appl. No. 839,841

[22] Filed July 8, 1969 [45] Patented Dec. 7, 1971 Y [73] Assignee Selbu Denkl Kogyo Kabushiki Kaisha Fukuoka, Japan [32] Priority Aug. 6, 1968 [33] Japan [54] MAGNETIC FIRING CIRCUIT FOR THYRISTORS 3 Clnlml, 12 Drawlng Flgl.

52 us. C1 323/4, 323/22 SC [51] Int. Cl G051 1/44, G05f 1/38 [50] Field of Search 321/25; 323/4, 9, 16-22, 34, 35

[56] References Cited UNITED STATES PATENTS 3" 76209 .1 Q2QX Primary ExaminerGerald Goldberg Attorney-Holcombe, Wetherill & Brisebois ABSTRACT: A novel control circuit for constant current applications. The circuit comprises a thyristor bridge which is tired by a magnetic amplifier. The DC output current of the thyristors flows through the feedback winding of the magnetic amplifier to cancel the effectof the input ampere-turns, and to keep the output current constant through the equal ampereturns characteristic of the conventional saturable reactor. Because thyristors are used in the output stage, the new circuit can control a large amount of output power with a device of light weight and small size. The principle of the new circuit is also effectively applied to parallel operation of multithyristors, in which individual currents can be adjusted to various desired values.

PATENTED DEC 7 l9?! SHEET & [1F 6 ill Ill] ll ITTI llll III! IIIT lxln Ill:

I III PATENIEU on: 1 gm SHEET 5 BF 6 MAGNETIC FIRING CIRCUIT FOR THYRISTORS BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a current-control system, and more particularly to a magnetic amplifier system for controlling the current output of a thyristor bridge.

2. Description of the Prior Art Control devices which can control a large amount of output power so as to maintain the output current constant have uses in many industrial applications, such as electrolysis, arc furnaces, electric power distribution, power supplies for various communication systems, etc. Attempts have been made to solve this problem by the use of the law of flux cancellation with opposing equal ampere-turns in the conventional saturable reactor. However, when the output power to be controlled is large, a saturable reactor sufficiently large to handle the control function may be too heavy, too large and too expensive.

SUMMARY OF THE INVENTION The present invention avoids the problem of an overly large saturable reactor and still uses the basic principle of the BRIEF DESCRIPTION OF THE DRAWINGS FIG. la is a first circuit embodying the invention for providing a DC output.

FIG. 1b is a second circuit embodying the invention for providing an AC output.

FIG. 2 isa block diagram of the new circuit with an induc-v tive DC load.

FIG. 3 is a curve showing the control characteristics obtained from the circuit shown in FIG. Ia, with the broken line indicating the theoretical result and the solid lines indicating the experimental results obtained.

FIG. 4 is a curve showing experimental results of the voltage current relation of the output mean values.

FIGS. 50 and 5b show new combination circuits with a current bypass element in the feedback path, FIG. 5a being for use with a DC output and 5b for use with an AC output.

FIG. 6 is a representation of an oscillogram showing the transient response of the circuit for an inductive load.

FIG. 7 is a schematic diagram of a circuit embodying the invention for parallel operation of thyristors.

FIG. 8 is curves showing experimental control characteristics of an individual branch circuit with the circuit of FIG. 7.

FIG. 9 shows an oscillograrn of the individual currents corresponding to the mark in FIG. 8.

FIG. 10 is a curve as given in FIG. 8 under different winding conditions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1a is a schematic diagram of one embodiment of the invention using a magnetic firing circuit for firing a thyristor bridge to provide DC output power. A control current I is provided to input terminals I and 2 of a self-saturating mag netic amplifier 3. The current I enters through input resistance R to control windings 4 which have N turns. The magnetic amplifier uses the control current to derive firing signals on lines 5 and 6. The firing signals are applied to thyristor bridge 7 to fire thyristors 8 and 9.

Transformer 10 provides an output power waveform efrom a secondary winding ll which is applied across the bridge, at nodes 12 and 13. When thyristors 8 and 9 are fired, the bridge output voltage appears in series across an output inductance L, an output resistance R,, and an output feedback winding 14 of the magnetic amplifier. Winding I4 has N, turns and carries an output current 1,.

Windings4 and 14 are wound in opposition to each otherso that when l,N,=l,N the resulting magnetic fluxes in the amplifier core cancel each other.

In practice of course, the. fluxes do not completely cancel each other. Instead, a small error flux remains, analogous to the small error voltage at the input to an operational amplifier when provided with input and feedback impedance- As with ing 17 having Np turns. This feedback is adjusted by choice of Np to make the open-loop gain approach infinity, thereby im-. proving the linearity of the transfer characteristic.

Winding .18 and bias source 19 are used to adjust the zero point ,of the magnetic amplifier. An AC source 20 drives a primary winding 21 of transformer 10. A secondary, winding 22 provides AC power to ,outputwindings 23 of the magnetic amplifier.

To explain theoperation of the .invention an analysis of a typical circuit as shown in FIG. vla will be given. If the effect of sampling at every half cycle is neglected, the transfer function of the magnetic amplifier, G, can be described as follows:

the Laplace transformedmean value of the output voltage of I the magneticamplifier, neglecting theexciting current component in the gate winding;.K is the voltage gain; and T is the time constant of the ma'gneticjamplifier.

When the number of turns ofthe gate and lhfiicontrolwinding of the magnetic amplifier are designatedas N and N respectively, the following ,well-knownrelationship exists with respect to the gain-bandwidth product or the dynamic voltage, gain:

From equations 1 and 2 is derived GM: 1 al e f KEM e 3+1 3 Defining K as the voltage gain of the thyristorcircuit, or the ratio of the mean output voltage .of the thyristor circuit to that of the magnetic amplifier, then n=E"./ where E", is the mean value-of the output voltage of the thyristor circuit. K is roughly determined by the voltage ratio of the AC supply to the thyristors and that to the magnetic amplifier.

The transfer function G of theload circuit of the thyristor is as follows:

E o( T1. +1

where E, (s) is the Laplace transformed mean voltage at the terminals of the load resistance and T =LIR is the time constant of the load circuit.

The DC outputcurrent I, controlled by the thyristors is fed through the-feedback winding, with N the number of turns. The polarity of the magnetomotive force N,,l, is opposite to that of the control ampere-turns NJ, as in a self-balancing magnetic amplifier. I

Considering the negative feedback effect as in a conventional saturable reactor, one obtains the block diagram of FIG. 2, in which R represents the control circuitresistance.

From FIG. 2 can be derived the overall transfer function G as follows:

If K approaches infinity by adjustment of the amount of positive feedback in the magnetic amplifier, as by adjusting N,,, then from equation 6 is derived o l N o f NG Equation 7 is the basic relation .for the operation of the new circuit. From this equation can be derived the characteristics of the device, as given in the following paragraphs.

The steady state voltage gain K with a DC input voltage E, can be derived for s approaching zero in equation 7 as follows: I(,,-=E,,IE,=N,,R,,/N,,R 8

which reduces to I f m YT, KB If the current transfer ratio K of the thyristors is used instead of the voltage gain K the time constant is obtained as follows:

if K ,,=1"5\d N,=N in equation 12, the same relation exists as in a saturable reactor with parallel gate windings and separate gate resistance R From equations ll and 12, it becomes evident that the time constant of the new device is inversely proportional to the current gain of the thyristor.

From equations 8 and ll, the gain-bandwidth product or the dynamic voltage gain. of the new circuit is obtained as follows:

s/T=2f c/ c' s1 13 The gain-bandwidth product derived in equation 13 becomes K times larger than that of equation 2.

The figure of merit of the dynamic power gain (i.e., the ratio of the power gain K,. to the time constant T) is obtained from equations 8 and ll as follows:

K /T=2f'N /N,-K -kf 14 where kf is the form factor of the output current. Since N /N, is very large (of the order of to 10) for the ordinary case, the figure of merit of the new circuit is 10 to 10 times larger than that of the conventional reactor, which is expressed as 4fkf.

It is evident from equation 7 that when this circuit is used with an inductive DC load, the transfer function has a second order characteristic equation. An oscillatory transient response to a step input change will appear when 5; lawn) 15 This response is similar to that of a saturable reactor with an inductive DC load. It is evident from equation 9 that the transfer characteristic of the new circuit is determined only by the turns ratio N /N independent of the core characteristics and the load impedance.

With the circuit elements as described in the following table:

TABLE 1 COMPONENTS USED IN THE EXPERIMENT Magnetic amplifier core-50% Fe-Ni Alloy (Sendelta) Toroidal core (0.05X35X25X5 mm.)

Magnetic.amplifier winding--400 turns (all windings have same number of turns, except for N Thyristor-2SF245 (2lRC40, lR) which is set forth by way of example only and not by way of limitation, certain experiments were conducted.

FlG. 3 shows the experimental transfer characteristics with the circuit elements described in'table l with the current gain set at 400 (N =400, N,,=l) and varying R,. The linearity of the transfer characteristic is excellent, as in the conventional selfbalancing magnetic amplifier, but the firing operation of the thyristor will be unstable for an extremely small input signal. In experiments, it was found that an input signal less than about 3 percent of that corresponding to the maximum output gave unstable firing. This defect can be completely eliminated by using a magnetic amplifier with a supply frequency of onehalf that of the thyristor. This system has been called a parametric phase shifter."

In FIG. 4 are given experimental results showing the constant current characteristics with changes in the output resistance. The voltage-current relations of the mean output values were measured, taking l, as a parameter. In these experiments it was observed that the current changed less than 0.1 percent as the output resistance R, was varied between the limits of 5 to 20 ohms.

The maximum output power was about 2 kilowatts in this experiment, and this maximum power was about l0 times the rating of the magnetic amplifier used in the gate circuit of the thyristor bridge. The ratio of the maximum output power to the rating of the magnetic amplifier increases with the ratio of the thyristor. Where the ratio is extremely large, however, the equal ampere-turns relation of equation 9 may be lost because of the small value of load impedance. ,This effect can be removed by reducing the peak of the output current, which effectively changes the output impedance. Either filtering the load current or increasing the rating of the gate magnetic amplifier will be effective.

When the ratio of the demand to the output current of the thyristor becomes larger with the V-A rating of the gate magnetic amplifier remaining unchanged, it will be necessary to bypass the feedback mmf (magnetomotive force). Various methods may be used for this purpose. As an example, DCCT (a Direct Current Current Transformer) and CT (a Current Transformer) are shown in FIGS. 5a and 5b. In this way the rating of the magnetic amplifier becomes independent of the ratio of the demand to the output current of the thyristor device.

The time constant for a pure resistive load can be calculated theoretically with equation 1 l as T=3 cycles for R =l 00 ohms, R,,=5 ohms, N,,=l, e =l00 volts RMS, and e,,,=30 volts RMS, with the circuit elements in table 1. The result was a little shorter than the measured value. This difference (less than 1 cycle) is due to the fact that the sampling effect transportation lag of the magnetic amplifier is neglected in equation l l.

For an inductive DC load as in FIG. la, the experimental results of the transient system response are given in FIG. 6, where R,.=l00 ohms, N =50, N,=l00, T,,=5.5Xl0 sec. and R,,=20 ohms. It will be observed from FlG. 5 that the response for the inductive loading is oscillatory, and that the result of equation 15 is therefore correct.

I and FIG. 1b is a diagram of a circuit generally similar to that of FIG. Ia, but slightly modified to provide an AC output to load Z, rather than a DC output to load L, and R as in FIG. la.

The principle of the constant current characteristics explained in the preceding sections can be easily extended to the parallel operation of multithyristors. Parallel operation is necessary to control large amounts of output power by several thyristors of low rating. This method of parallel operation provides a solution to the problem of the power limitation of a thyristor control device.

By arranging the n parallel circuits as shown in FIG. 7 (with reference to the circuit shown in FIG. lb) and by choosing arbitrarily the control ampere-tums of each magnetic amplifier, we have a complete parallel operation in which the burden of each thyristor can be selected according to its current rating. If the control ampere-turns of each magnetic amplifier are defined as N l N I WN I and branch currents as I then the following equation is obtained:

abuan where Ioi jT oi ci for l i 17 From equations 16 and 17 it is evident that the individual currents can be adjusted by selecting the ampere-turns N (i=1 ,2,....,n) arbitrarily. Especially, if N =N I =I (Fl ,2,.... ,n), equations 16 and 17 become as follows:

l,,.==l/nl,, (i=l,2,....,n) l8 N l =nN l 19 From equations 18 and I9, currents passed through the branch circuits have equal mean values independent of the internal resistance of the branches R R,-,,....,R,,,. In this case, the firing angles of the thyristors in each of the branch circuits are self-adjusting to equalize the mean values of the individual currents.

In FIG. 8 are shown the experimental results for n=3, N,=l, N,,=N =N ,--400, in which the mean value of the current of each branch circuit is measured as a function of the control current I, In the experiments, resistances R =0.5 ohms, R 1 ohm, and R -2 ohms were used, and the mean values of individual currents were found to balance each other. The waveforms of the branch currents corresponding to the mark in FIG. 8 are shown as in FIG. 9. For comparison, the measured currents without feedback (N,,=0) are shown in FIG. 10.

I claim:

l. A current-control system comprising:

A. a magnetic amplifier comprising first input terminal means, second input terminal means, and output terminal means, said first input terminal means being adapted to receive a current-control signal,

B. a semiconductor controlled rectifier current-control means adapted to receive electrical power and to control said power for application to a load by controlled firing of thyristor means therein,

C. means for connecting the output terminal means of the magnetic amplifier to the semiconductor controlled recti fier current-control means for controlling the firing of said thyristor means therein, and D. means for feeding back from the power output of the semiconductor controlled rectifier current-control means a negative feedback signal indicative of the current level of said power output for application to the second input terminal means, whereby the power output of the semiconductor controlled rectifier current-control means is controlled to supply a current proportional to the control signal. 2. A system according to claim 1 wherein said magnetic amplifier includes means for adjusting the open-loop gain of the amflifier to a very latrige value.

. A system accor mg to claim I wherein said semiconductor controlled rectifier current control means comprises a plurality of separate current-control systems operated in parallel.

i it k 

1. A current-control system comprising: A. a magnetic amplifier comprising first input terminal means, second input terminal means, and output terminal means, said first input terminal means being adapted to receive a currentcontrol signal, B. a semiconductor controlled rectifier current-control means adapted to receive electrical power and to control said power for apPlication to a load by controlled firing of thyristor means therein, C. means for connecting the output terminal means of the magnetic amplifier to the semiconductor controlled rectifier current-control means for controlling the firing of said thyristor means therein, and D. means for feeding back from the power output of the semiconductor controlled rectifier current-control means a negative feedback signal indicative of the current level of said power output for application to the second input terminal means, whereby the power output of the semiconductor controlled rectifier current-control means is controlled to supply a current proportional to the control signal.
 2. A system according to claim 1 wherein said magnetic amplifier includes means for adjusting the open-loop gain of the amplifier to a very large value.
 3. A system according to claim 1 wherein said semiconductor controlled rectifier current control means comprises a plurality of separate current-control systems operated in parallel. 